Substrate is a critical component for a semiconductor package, which usually serves as a chip carrier whose one side is mounted with at least one semiconductor chip that is electrically connected to conductive traces formed on the substrate. On an opposite side of the substrate there are also formed a plurality of conductive traces that are electrically coupled to the conductive traces on the side having the chip by means of conductive vias penetrating through the substrate. BGA (ball grid array) package is characterized in bonding an array of solder balls to the side opposite to the chip-mounting side of the substrate and electrically connecting the solder balls to the conductive traces on the substrate, and the solder balls can be further bonded to an external device such as printed circuit board (PCB). By this arrangement, the chip is electrically coupled through the conductive traces on both sides of the substrate and the solder balls to the PCB, such that the solder balls serve as input/output (I/O) connections for communicating the semiconductor package and the PCB and thereby play an important role.
The substrate is primarily composed of a core layer, a plurality of conductive traces formed on the core layer, and a solder mask applied over the conductive traces. In exemplification of the ball-bonding side of the substrate, once the core layer is prepared and made of a resin material such as epoxy resin, polyimide resin, BT (bismaleimide triazine) resin, FR4 resin, or FR5 resin, a film of copper or copper alloy is coated on a surface of the core layer and subject to a patterning process (including exposing, developing and etching, etc) to form the plurality of conductive traces in the core layer, each conductive trace having a terminal. Then, the solder mask is coated on the conductive traces while exposing the terminals. The exposed terminals form solder pads where the solder balls are bonded.
The solder pads are defined by a plurality of openings formed through the solder mask and corresponding to the terminals of the conductive traces. As shown in FIG. 5, a SMD (solder mask defined) pad 10 formed on the core layer 11 of the substrate 1 includes a predetermined part of the terminal 12 exposed via a corresponding opening 130 of the solder mask 13 which is sized smaller than the terminal 12. Solder is deposited over the pad 10 and reflowed to form a solder ball (or bump) 14 that is thus bonded and electrically connected to the substrate 1. Another type of solder pad is so-called NSMD (non-solder mask defined) pad 15, as shown in FIG. 6, formed by a corresponding opening 131 of the solder mask 13 which is sized larger than the terminal 12 to expose the entire terminal 12 and a portion of the core layer 11 around the terminal 12. As a result, the NSMD pad 15 includes the entirely exposed terminal 12 and the exposed portion of the core layer 11, allowing the solder ball 14 formed thereon to completely cover the terminal 12.
The SMD pad 10 for solder joint however renders significant drawbacks. The solder ball 14 mounted on the SMD pad 10, as in contact with relatively smaller area of the terminal 12, would easily lead to cracks at a neck or root portion thereof (as indicated by corrugated lines in FIG. 5) when the solder ball 14 is subject to external impact or shear force. With regard to the NSMD pad 15 on which the solder ball 14 covers the entire terminal 12, however, during a reflow process for forming the solder ball 14, solder deposited over the terminal 12 would melt under a high temperature and collapse to contact both the terminal 12 and the exposed portion of the core layer 11 around the terminal 12, as shown in FIG. 6. As a result, the fabricated solder balls 14 on different NSMD pads 15 may be tilted or hardly controlled with the height thereof depending on different degrees of solder collapse over the exposed core layer 11.
Another issue relates to resin flash during an encapsulation process for encapsulating the chip on the substrate in the use of a resin compound such as epoxy resin. This encapsulation process is usually performed before forming solder balls on the substrate. In other words, the substrate with its solder pads “naked” or exposed is placed in a cavity of a mold where the resin compound is injected to cover predetermined area (such as the chip-mounted side) on the substrate. However, it may occur that the mold, which is supposed to tightly abut against the side with solder pads on the substrate where no encapsulation is required, is not perfectly clamping this side of substrate and thus makes the resin compound leak or flash to this pad-forming side and contaminate the naked solder pads. In this case, the SMD pads and NSMD pads would all be undesirably contaminated by resin flash, while the SMD pads may encounter more serious flash over the terminals of conductive traces than the NSMD pads because the NSMD pad 15 (FIG. 6) has a recessed clearance (corresponding to the exposed portion of the core layer 11) between the solder mask 13 and the terminal 12, which recessed clearance may somewhat impede the movement of resin flash.
U.S. Pat. No. 6,201,305 discloses a type of solder pad that may increase contact area between the pad and a solder ball or bump mounted on the pad. As shown in FIG. 7, this solder pad 16 is fabricated by firstly etching the trace terminal 12 from the periphery toward the center of the terminal 12 to be shaped as a starfish having a plurality of radially-arranged spokes, and then forming an opening 132 through the solder mask 13 for exposing a predetermined portion of the starfish-shaped terminal 12 and a predetermined portion of the core layer 11 located between two adjacent spokes. Thereby, a solder ball or bump (not shown) deposited on the solder pad 16 comes into contact with both the exposed portions of the terminal 12 and the core layer 11, such that contact area between the pad 16 and the solder ball (or bump) is increased, which thereby secures the solder ball (or bump) strongly bonded to the pad 16 and not easy to crack in response to external impact or shear force.
However, a problem rendered by the above solder pad 16 is the difficulty in arranging such solder pads 16 in a fine pitch manner. For the fine pad pitch arrangement, which is advantageous for high chip integration, adjacent solder pads 16 or terminals 12 need to be closely situated and spaced from each other by a very small pitch distance, even as small as 0.5 mm, and this would make the particular starfish shape of the terminals 12 hard to be perfectly formed by the etching technique that is actually difficult to be performed from the periphery of terminals 12 very closely adjacent to each other, leading to undesirably etching results. The undesirable etching results may caused by, for example, inaccurate etching that forms imperfect starfish shape of the terminal 12 whose predetermined portion may not be completely exposed via the opening 132 of the solder mask 13; over etching that would etch out too much part of the terminal 12, making the remaining part of terminal 12 that is even exposed via the opening fail to provide sufficient bonding strength with the solder ball (or bump) formed thereon; or insufficient etching that may leave the etching-out part of terminal 12 not able to be exposed via the opening 132, such that the predetermined portion of the core layer 11 intended to be exposed via the etching-out part of terminal 12 cannot be exposed but remains covered by the solder mask 13, which also degrades the bondability between the solder pad 16 and the solder ball (or bump).
FIG. 8 illustrates another type of solder pad 17 similarly formed by etching, for which the trace terminal 12 is etched from the periphery toward the center thereof to form a cross that may serve as an air vent when the solder pad 17 bonded with a solder ball or bump (not shown) undergoes a reflow process, and then a predetermined portion of the core layer 11 exposed via the cross and four portions of the terminal 12 are exposed by an opening 133 formed through the solder mask 13. Besides the drawbacks discussed above for the solder pad 16 shown in FIG. 7, the present solder pad 17 of FIG. 8 further encounters a problem that the cross or air vent occupies relatively more area exposed via the opening 133, making the relatively less exposed portions of terminal 12 not able to firmly bonded with the solder ball (or bump), such that the bonding in-between may be easily destroyed by external force or impact.
Moreover, the solder pads shown in FIGS. 7 and 8 are easily subject to resin flash. During a molding process performed prior to ball implantation, a resin compound e.g. epoxy resin commonly used to encapsulate mounting components (such as chip, bonding wires, etc.) on the substrate may undesirably flash to the exposed solder pads (especially the exposed portion of the core layer) in the absence of a flash-preventing mechanism provided on the side of the substrate having the pads. The contaminated pads are not able to be perfectly bonded with solder balls or bumps, thereby damaging the quality of electrical connection between the pads and corresponding solder balls (or bumps).
Therefore, in response to the above drawbacks, the problem to be solved herein is to provide a substrate formed with pads for bonding solder balls or bumps, which can arrange the pads in a fine pitch manner, secure bondability between the pads and solder balls, and provide good resin-flash control.